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Using SELECT in SELECT

The result of a SELECT statement may be used as a value in
another statement. For example the statement
SELECT continent FROM world WHERE name = 'Brazil'
evaluates to 'South America' so we can use this value to
obtain a list of all countries in the same continent as
'Brazil'

List each country in the same continent as 'Brazil'.

SELECT name FROM world WHERE continent =(SELECT continent
FROM world WHERE name ='Brazil')

SELECT name
FROM world WHERE continent =(SELECT continent FROM world WHERE name ='Brazil')

Alias

Some versions of SQL insist that you give the subquery an alias. Simply put AS somename after the closing bracket:

SELECT name FROM world WHERE continent =
(SELECT continent FROM world WHERE name='Brazil') AS brazil_continent

Multiple Results

The subquery may return more than one result - if this happens the query above will fail as you are testing one value against more than one value.
It is safer to use IN to cope with this possibility.

The phrase (SELECT continent FROM world WHERE name = 'Brazil' OR name='Mexico')
will return two values ('North America' and 'South America'). You should use:

SELECT name, continent FROM world
WHERE continent IN
(SELECT continent FROM world WHERE name='Brazil'
OR name='Mexico')

List each country and its continent in the same continent as 'Brazil' or 'Mexico'.

SELECT name, continent FROM world
WHERE continent IN(SELECT continent
FROM world WHERE name='Brazil'OR name='Mexico')

Subquery on the SELECT line

If you are certain that only one value will be returned you can use that query on the SELECT line.

Show the population of China as a multiple of the population of the United Kingdom

SELECT
population/(SELECT population FROM world
WHERE name='United Kingdom')FROM world
WHERE name ='China'

Operators over a set

These operators are binary - they normally take two parameters:

= equals
> greater than
< less than
>= greater or equal
<= less or equal

You can use the words ALL or ANY where the right side of the operator might have multiple values.

Show each country that has a population greater than the population of ALL countries in Europe.

Note that we mean greater than every single country in Europe; not the combined population of Europe.

SELECT name FROM world
WHERE population >ALL(SELECT population FROM world
WHERE continent='Europe')